New pulse forming network power supply for the Caltech Spheromak Formation Experiment

Past experiments on the Caltech Spheromak Formation Experiment produced plasmas with a 120 $\mu$F capacitor bank charged to 4--7 kV and supplying a 150 kA peak current with a FWHM of $\sim$10 $\mu$s. We recently completed construction of a pulse forming network (PFN) having two parallel sections of five 120 $\mu$F capacitors, designed to produce a 150 kA peak current pulse with FWHM of $\sim$50 $\mu$s. Preliminary experiments using only the PFN show the effect of the five--fold increase in current pulse length on the collimated jet precursor to spheromak formation: the jet extends to 55 cm, a length $\sim$1.5 times that previously seen, and can develop two visible twists upon onset of the kink instability. Future experiments will use the 120 $\mu$F capacitor bank for plasma breakdown and the PFN for sustainment. A recently constructed capacitively coupled probe and axial 60--channel magnetic probe array will be used to study critical ionization velocity limited collisions between the magnetized jet and a neutral target gas cloud.